1. Field of the Invention
The present invention relates to optical devices which utilize light beams to identify the coordinates of an object intersected by the light beams.
2. Prior Art
The use of optical touch panel devices as I/O terminals for data processing systems is well known in the prior art. A typical optical touch panel device includes a CRT with a transparent layer disposed over its screen. An optical assembly which includes light generating sources, photodetector, reflectors, etc. is placed relative to the translucent layer. The optical assembly generates a plurality of light beams which criss-cross the surface of the translucent layer. If there is no object blocking the light rays, the light travels from the light generating source uninterrupted to the photodetector producing a continuous photodetector current. If an object is placed within the light plane formed by the rays, the object blocks the light beam from contacting the photoconductor. This causes a discontinuous photodetector current. The discontinuity in photodetector current is called "light dropout" which indicates the presence of an object.
A triangulation algorithm or similar technique uses the light dropout phenomenon to calculate the precise location of the object. In case of a touch to the CRT surface by a pointer to indicate a location on said CRT surface, the triangulation technique can be used to identify the precise location where the contact occurs. A more detailed description of these prior art devices is set forth in the following U.S. Pat. Nos.: 3,553,680 (Cooreman), 3,613,066 (Cooreman et al), 3,898,445 (Macleod), 4,294,543 (Apple et al), 4,420,261 (Barlow et al), 4,558,313 (Garwin et al), 4,710,759 (Fitzgibbon et al) and 4,710,758 (Mussler et al).
A problem which is common to the above-described optical devices, and not addressed by the prior art, is system degradation caused by optical aberrations. Optical aberrations may include a discontinuity in one or more of the reflective surfaces, a scratch on the protective plastic shield that encloses the optical emitter/detector and/or reflective surfaces, debris, etc. As with the valid touch to the CRT surface, the optical aberration prevents the light beam from reaching the photodetector. This unwelcome intrusion causes light dropout called "aberration dropout" in the photodetector current. These aberration dropouts appear as valid touches to the system. In addition, most systems are only capable of detecting one touch at a time. Since an "aberration dropout" is processed as a valid touch, a valid touch may go undetected causing a catastrophic system failure.
A straightforward solution would be to make sure that the reflective surfaces are free of aberrations. This can be achieved through rigid control of manufacturing and maintenance processes. This solution may not be practical since it may unnecessarily increase the cost of the system. Also, such terminals may be used in uncontrolled environments (such as public information access applications) in which it may not be possible to prevent damage to or degradation of the reflective surfaces.